SSVs are used in production strings to control the well. They are mounted in the string and are hydraulically controlled from the surface. Typically a control line runs parallel to the production string and is connected to the SSV housing. Applying pressure moves a piston that is connected to a flow tube. The flow tube is pushed against a closure spring by the piston. The flow tube also engages a flapper to rotate it 90 degrees so that the flow tube can advance as the open flapper is now outside the flow tube. The housing has a seat and the flapper is biased by a torsion spring against the seat. The movement of the piston to urge the flow tube to move winds the torsion spring and compresses the closure spring at the same time. When pressure is removed or lost from the control line, the closure spring pushes the flow tube and interconnected operating piston against the hydrostatic pressure in the control line so that as the flow tube rises the torsion spring is enabled to rotate the flapper into contact with the seat.
If a problem occurs within the SSV it usually means that it has to be pulled with the production string. Variations involving balance control lines or pressurized chambers in the SSV housing have been developed to allow offsetting of hydrostatic pressure since the hydrostatic pressure in the main control line is offset and that allows a smaller closure spring to close the valve without having to also overcome the hydrostatic pressure in the control line.
Problems could occur in the hydraulic actuation system such as a control line leak or an operating piston seal leak, for example. Dual operating control systems have been developed so that one operates the SSV while the other system is isolated until needed. In these systems, each control system had its own control line and operating piston where both operating pistons were engaged to the flow tube. In order not to burden the single closure spring with the added hydrostatic pressure from two parallel control lines the system that is offline is isolated with a rupture disc so that the hydrostatic pressure above the disc is not felt by the closure spring until the disc is broken, generally by raising tubing pressure.
However, in subsea systems the delivered pressures are controlled and can't be arbitrarily raised to affect a switch to the backup control system by raising the pressure in the system above the normal operating range. This condition in subsea systems has been addressed by the present invention. There are the two control lines each going to a discrete independent operating piston. Each piston is coupled to a rod and the two rods interact. The rod associated with the piston where control line pressure is applied is free to move to operate the SSV in the normal manner. The movement of the first piston and its associated rod results in support for the other rod in a variety of ways explained below. The result is that the rod associated with the non-pressurized system has the hydrostatic pressure in its control line isolated from the closure spring. Removing applied pressure from the control lines lets the system go back to neutral so that either of the two redundant systems can be thereafter activated. Those skilled in the art will gain a better understanding of the invention from the description of the preferred embodiment with the associated drawings that appear below with the understanding that the claims define the full scope of the invention.